Furnace support for resinoid preforms

ABSTRACT

The invention relates to a support used in combination with a resinoid preform for maintaining the shape of the preform as it is cured in a furnace. More particularly, the invention relates to a furnace batt used to support a preform of a resinoid grinding wheel while the wheel is cured in a furnace at elevated temperature, usually over prolonged period of times. Broadly, the invention comprises a substrate which conforms very closely to the shape of the preform. The top and bottom surface of the substrate is coated with a hard metal coating. Preferably the exposed surface of the coating has a textured or roughened surface. The hard metal coating is, in turn, covered with a fluorocarbon composition. In the preferred embodiment an aluminum substrate is coated with molybdenum, the latter having a textured surface. A fluorocarbon coating is applied over the molybdenum.

United States Patent [72] Inventors Gardner E. Alden Fayville; George M. Herterick, Westboro, Mass. [21] Appl. No. 874,610' [22] Filed Nov. 6, 1969 [45] Patented May 11, 1971 [73] Assignee Avoo Corporation Cincinnati, Ohio [54] FURNACE SUPPORT FOR RESINOID 1,675,119 or /19 2 3 I Maiden Attomeys-Charles M Hogan and Abraham Ogman ABSTRACT: The invention relates to a support used in combination with a resinoid preform for maintaining the shape of the preform as it is cured in a furnace. More particularly, the invention relates to a furnace batt used to support a preform of a resinoid grinding wheel while the wheel is cured in a furnace at elevated temperature, usually over prolonged period of times. Broadly, the invention comprises a substrate which oonfomis very closely to the shape of the preform. The top and bottom surface of the substrate is coated with a hard metal coating. Preferably the exposed surface of the coating has a textured or roughened surface. The hard metal coating is, in turn, covered with a fluorocarbon composition. ln the preferred embodiment an aluminum substrate is coated with molybdenum, the latter having a textured surface. A fluorocarbon coating is applied over the molybdenum.

Patented May 11, 1971 m n 8 a w a a I J EAE E m n RM M Mm w A m w 3 a, j w m 4 I l um um 1 "ml 0 i w m m w m w" 8 2 1 FURNACE SUPPORT FOR RESINOID PREFORMS DEFINITION For purposes of this discussion the term fluorocarbon is defined to mean polytetrafluoroethylene, polytrifluorochloroethylene, polymers, and telomers containing tetrafluoroethylene and trifluorochloroethylene; these are commonly sold under the common trademarks of KeI-F, Vydax, and Teflon.

Additionally, a hard material shall be construed to mean a material having a hardness of at least 60 measured on the Rockwell N15 scale.

BACKGROUND OF THE INVENTION The discussion will be covering the use of the invention with reinforced resinoid grinding wheels, but it will be quite evident that its usefulness will extend to any other form of grinding v wheels or resinoid products that are manufactured under conditions similar to the manufacturing processes forreinfor ced grinding wheels. Additionally, for purposes of illustration, the discussion will be limited to grinding wheels in the form of discs, such as cutoff blades; depressed hub configurations, such as the type 27 and 28 shape (see American Standards Association Bulletin B7.I-'l964, directed to abrasive products, page l2); and hat wheels manufactured and sold under the brand name of Koolie I-Iat" owned by the assignee of this invention.

Reinforced resinoid wheels are first formed into a preform" by pressing an abrasive composition comprising abrasive grains and a resinoid bond in combination with, typically, a glass fabric reinforcement, into a desired shape. The preform is placed on a batt. The batt is configured to conform to the shape of the preform. Alternate layers of batts and preforms are arrrmged in a vertical stack of about preforms. The stack is placed in compression. The compressed stack is then placed into an oven at elevated temperatures for a prolonged length of time until the resinoid bond is fully cured. The stack is removed from the furnace, cooled, and the grinding wheels separated from the batts.

It is at the point of separation that the problem solved by the present invention occurs. While the entire surface area of the preform is under compression it is known that the region around the mounting hole of the grinding wheel, particularly in the depressed center configurations, are stress concentration regions. The cured wheels tend to stick to the batts in the vicinity of the mounting holes at the stress concentration regions.

In prior practice the grinding wheels were separated from the batts by bashing the combination of batt and grinding wheel on a soft surface. Most of. the times aclean separation will result at the first blow. Many times, however, successive blows are required.

Many times the grinding wheel will separate from the batt only after a portion of the wheel breaks away from the wheel and remains on the batt. A defective wheel results.

The problems that existed prior to the invention are manifold. For one thing, the resins used to bond abrasive grit are adhesives. Whereas the resin needs to bond the abrasive grit into a strong mechanical structure, it is also necessary to prevent the same adhesive bond from adhering or sticking to the batt.

The pounding that was heretofore required to separate the adhesive from the batt deforms the batt. After a period of time the batt mustbe returned to the machine shop and reconditioned where a portion of the top and bottom surface of the batt is removed. The remaining portion is reshaped to conform with the grinding wheel configuration. After three reconditioning cycles, the batt must be discarded because its thickness is reduced to the point where continued use will result in the batts being warped by the alternate heating and cooling cycles.

Heretofore the sticking problem has been relieved by treating the top and bottom surface of a batt with a silicone parting agent. The surfaces of the batt needto be retreated in'from 2 to 5 cycles through the furnace, however.

In prior art batts the top and bottom surfaces of the aluminum substrate contain pointed protuberances. The protuberances preferably form a helix or concentric circles as these are most easily formed in a conventional machining process.

The pointed peaks are provided to reduce the contact area between the batt and the preform to lessen materially the areas on which sticking can take place between the preform and the batt. This did not solve the problem.

Reinforced resinoid grinding wheels are high volume products. Thousands of batts are used daily. It is impossible to schedule retreatment. The need to retreat a batt first becomes evident when a defective wheel caused by sticking is produced.

To further emphasize the gravity of the problem in the production of these grinding wheels, the cost of retreating a batt is from one-fourth to one-sixth of the cost of producing a grinding wheel. This ratio does not include the cost of reconditioning as mentioned above. About 30 to 40 percent of the inventory is being retreated or reconditioned at any time; this is an added cost.

In general, a batt is made from an aluminum material because it is durable, easily machined, and inexpensive. At-

tempts have been made to eliminate the sticking problem by coating aluminum batts with fluorocarbon materials. The time between retreatment was improved slightly, but the coating quickly lost its effectiveness due to abrasive action. The added cost of handling fluorocarbon material and of the fluorocarbon material itself offset any cost advantage that resulted.

A metallic coating, including a molybdenum coating, overthe aluminum was also ineffective.

It is, therefore, an object of the invention to provide a furnace support for resinoid preforms which avoids difficulties and disadvantages of prior art devices.

It is yet another object of the invention to provide a furnace support for preforms which has an extended life between retreatment and/or reconditioning to the point of making the cost of these procedures an insignificant factor in the cost of manufacturing reinforced resinoid grinding wheels in particular.

It is still another object of the invention to provide a furnace support for resinoid preforms which significantly eases the handling and separating of cured preforms and reinforced resinoid grinding wheels in particular from the support structure.

It is yet another object of the invention to improve the ease of separating reinforced resinoid grinding wheels from batts to the extent that this step may be handled by automated equipment.

It is yet another object of the invention to provide a batt for reinforced resinoid grinding wheels which reduces grinding wheel rejects caused by sticking to a very low level.

In accordance with the invention, a support for resinoid preforms cured under pressure comprises a metal substrate with its mating surface coated with a material having a minimum 60 hardness on the Rockwell N15 scale. The exposed surface of the hard coating is textured or rough. The hard material is in turn coated with fluorocarbon material.

The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment whenread in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a stack of alternate layers of a type' 1 flat reinforced resinoid grinding wheel and conforming batts;

FIG. 2 is a section of a Koolie I-Iat" type grinding wheel;

FIG. 3 is an enlarged partial sectional representation of a type 27 depressed center wheel situated between'two batts; and

FIG. 4 is a gross enlargement of a portion of the FIG. 3 diagram to show details of the invention.

Referring to FlG. 1 of the drawings, there is shown a stack of flat preforms 10, either grinding wheel or other resinoid products, prepared for curing in a furnace. The stack comprises a bottom plate lll on which a batt 12 is positioned. The stack comprises alternate layers of batts l2 and grinding wheel preforms l3.

The stack 10 is topped by a pressure plate 14. The entire stack 10 is maintained in compression by a spring-loaded bolt assembly R6. v

Referring to FIGS. 2 and 3, the numerical symbols l7 and 18 identify the stress concentration regions for the depressed center wheels 21 and 22 respectively. in general, even in the case of the flat wheel, the stress concentration points are in the region adjacent to the mounting hole. Virtually all of the problems associated with separating the cured preform from the batt occur in this region.

Referring to FIGS. 3 and 4 of the drawings, the improved batt 12 comprises a substrate 19 conforming in shape to the grinding wheel which it is designed to support. In the FIG. 3 illustration, it is seen that the substrate 19 conforms to the shape of a depressed center wheel. The substrate 19 is made preferably of aluminum because aluminum is ductile, easily machined, and inexpensive. In theory, the substrate may be made from any material, such as a ceramic or another metal so long as the substrate can withstand the curing temperatures. Aluminum is also desirable because it conducts heat readily and avoids localized hot spots.

The substrate 19 is coated with a hard material having a hardness of at least 60 on the Rockwell Nl5 scale. The preferred material for the coating 27 is molybdenum; the preferred process of coating the substrate for ease and economy is a thermal spray process. The thermal spray process may be a metallizing process using chemical combustion or a plasma spray process using high temperature plasma.

Thermal spraying has an added advantage of inherently producing a rough or textured surface. The surface texture is sometimes described as a knobby surface. ln the thermal spray art is is described as a laminar structure of interlocking and overlapping material. This is an important considerations with regard to the fluorocarbon coating 29 as will be discussed hereinafter. The surface roughness needs to be at least 100 microinches for practical effectiveness.

Molybdenum is the first choice because it is hard. It has excellent adhering properties to aluminum and is easily applied by means of thermal spraying processes.

The bond strength between the hard coating (molybdenum) and the substrate (aluminum) is another important property. The bond is stressed when the fluorocarbon coating is applied and sintered. Where the bond stress was below 4,000 p.s.i., separation occurred between the substrate and the bond material due primarily to the difference is coefficient expansion between the several materials. The bond is also stressed each time a grinding wheel is separated from a batt. The stress increases as the effectiveness of the fluorocarbon coating decreases. Where the bond strength between the hard material and substrate exceeds 4,000 p.s.i., the hard coating remains intact for the entire useful line of the fluorocarbon.

The preferred thickness of the molybdenum is 0.002 inches. A lesser thickness has a deleterious effect on the perfonnance of the batt; a greater thickness is wasteful. A lesser thickness is usually not continuous. Openings through the hard coating allow abrasives and resin to penetrate to the substrate tending to separate the hard material from the substrate. An alternate material for the coating 27 are nickel-chromium alloys.

The important consideration in selecting a coating 27 material is that it provides a good bond to the substrate and the fluorocarbon material that it be hard as defined herein and that it be easily applied.

The exterior surface 28 of the coating 27 is in turn coated with a continuous sintered fluorocarbon material 28 of the type defined previously to a depth of 0.002 inches to assure continuity. Fluorocarbons are selected because they are stable at the curing temperatures of most of the common resinoid materials. The fluorocarbons are known to be very inert. They are known to have nonstick properties to virtually all material.

In fact, special preparations are required to make fluorocarbons adhere. The preferred fluorocarbon is marketed under the brand name Teflon S. It is placed on the surface and sintered in an oven following procedures recommended by the Teflon S supplier.

A grinding wheel separates freely from a continuous fluorocarbon coating. The separation may be effected with automated equipment.

The hard material without the fluorocarbon coating 29 is known to be unsuitable, particularly for grinding wheels. A fluorocarbon coating 29 without the hard intermediate coating is also known to be unsuitable. The combination of a fluorocarbon and a hard coating, such as molybdenum, on an aluminum substrate does work well.

Batts prepared in the manner described containing a molybdenum coating 27 and a sintered fluorocarbon coating 29 have undergone over a hundred curved cycles with very little noticeable wear. On the basis of these observations it is estimated that the fluorocarbon-molybdenum-aluminum batt will not require reconditioning. While it is not possible to state with certainty when such batts will require retreatment, it is estimated that such retreatment will not occur until after 200 cycles. While the cost of preparing the fluorocarbon, molybdenum, and aluminum batt is greater than any of the prior preparations, when the cost is amortized over 200 cycles it becomes a negligible factor in the overall cast of preparing grinding wheels. Additionally, where previously about 30 percent of the batts is inventory were being retreated or reconditioned, at great cost, it is estimated that only 1-3 percent will be in this unproductive state, as a maximum.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims:

We claim: g

l. A furnace support for resinoid preforms comprising:

an aluminum substrate shaped to conform to the preform;

a continuous molybdenum coating on the substrate and conforming to thesurface, said molybdenum surface having laminar structure of interlocking and overlapping material; and

a sintered fluorocarbon coating covering the molybdenum surface.

2. A support structure as defined in claim 1 in which the thicknesses of the coatings are at least 0.002 inches.

3. A support structure as defined in claim 2 wherein the surface roughness of the molybdenum coating is microinches minimum and the bond strength between the molybdenum and substrate exceeds 4,000 p.s.i.

4. A furnace support for resinoid preforms comprising:

a substrate conforming to the shape of the preform;

a continuous metal coating having a hardness exceeding 60 on the Rockwell N15 scale covering at least the top and bottom surfaces of the substrate, the exposed surface of said coating being roughened; and

a continuous fluorocarbon coating covering the hard metal.

5. A support structure as defined in claim 4 in which the hard metal is molybdenum. I

6. A support structure as described in claim 4 in which the exposed surface is a laminar structure of interlocking and overlapping material.

7. A support structure as described in claim 4 in which the fluorocarbon coating is a sintered coating.

8. A support structure as described in claim 4 in which the hard material is molybdenum, the exposed surface is a laminar structure of interlocking and overlapping material having a roughness of at least 100 microinches, and the fluorocarbon coating is a sintered coating.

9. A support structure as defined in claim 8 in which the substrate is aluminum.

223 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatenrNo. 301 Dated May 11, 1971 Inventor(s) Gardner E. Alden It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Cblumn 3-, line 40, for "considerations" read--consideration--: line 52, for "is" read--in--; line 58, for "line" read--life--;

Column 4, line 27, for "cast" read--cost---;

line 29, for "is" read --in--.

Signed and sealed this 19th day of October 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOITSCHALK Acting Commissioner of Patents Attesting Officer 

2. A support structure as defined in claim 1 in which the thicknesses of the coatings are at least 0.002 inches.
 3. A support structure as defined in claim 2 wherein the surface roughness of the molybdenum coating is 100 microinches minimum and the bond strength between the molybdenum and substrate exceeds 4,000 p.s.i.
 4. A furnace support for resinoid preforms comprising: a substrate conforming to the shape of the preform; a continuous metal coating having a hardness exceeding 60 on the Rockwell N15 scale covering at least the top and bottom surfaces of the substrate, the exposed surface of said coating being roughened; and a continuous fluorocarbon coating covering the hard metal.
 5. A support structure as defined in claim 4 in which the hard metal is molybdenum.
 6. A support structure as described in claim 4 in which the exposed surface is a laminar structure of interlocking and overlapping material.
 7. A support structure as described in claim 4 in which the fluorocarbon coating is a sintered coating.
 8. A support structure as described in claim 4 in which the hard material is molybdenum, the exposed surface is a laminar structure of interlocking and overlapping material having a roughness of at least 100 microinches, and the fluorocarbon coating is a sintered coating.
 9. A support structure as defined in claim 8 in which the substrate is aluminum. 